1. Field of the Invention
This invention relates to protective devices for dynamoelectric machines and in particular to a weldable spark gap device for overvoltage protection of a synchronous motor.
2. Description of the Prior Art
Certain protective features are generally included within a motor or as part of a motor controller to prevent damage to the motor resulting from unusual operating conditions. For example, where an increase in voltage will cause damage to electric or mechanical apparatus, it is necessary to use some form of overvoltage protection.
A serious condition arises during the starting operation of a synchronous motor which requires overvoltage protection for the field winding. Since the synchronous motor is not inherently self-starting, it must be brought up to speed by some auxiliary means, and then connected across the line. Conventional synchronous motor designs employ pole mounted amortisseur windings and rely on induction motor principles for starting and accelerating the motor to synchronous speed. A.C. voltage applied to the stator windings creates a rotating magnetic field, and a very high voltage on the order of several thousand volts will be induced into the field circuit by transformer action if the field circuit is open circuited. The rapid build-up of this high voltage on the field circuit causes flashing over in the field winding which results in burning, shorting and grounding. The field winding must therefore be short circuited through a resistor or otherwise protected to limit the voltage across the field winding to a safe value.
The overvoltage condition will also occur during synchronous operation if the machine loses its DC excitation and pulls out of synchronism. The synchronous motor starts from stand still and accelerates to synchronous speed. When the motor approaches synchronous speed the field circuit is closed to a DC supply and the field discharge resistor is disconnected from the field. The rotor then turns in synchronism with the uniform rotating magnetic field produced by the stator winding. The direct current field excitation produces north and south poles on the rotor which rotate at a synchronous speed and are locked in synchronism with the synchronous rotating field of the stator. If for some reason the machine loses its DC excitation, it will pull out of synchronism and revert back to its transient starting characteristics. The high voltages will be impressed across the field but with the field discharge resistor disconnected. In this situation extensive damage to the rotor circuit will result if no protection is provided.
Synchronous machine protection has been limited primarily to overload and overvoltage protection of the stator winding. Some attempts have been made, however, to protect the rotor field winding with conventional, self-restoring spark gap devices mounted between collector rings. Conventional spark gap devices are well known in the art and are in general usage. In operation they are very similar, if not equivalent, to devices called lightning arrestors, the purpose of each being to protect electrical equipment from damage due to excessive electrical potentials which may be induced by lightning or other causes. In general, such devices comprise a pair of electrodes which are spaced apart to form a small gap. The gap prevents the flow of current between the electrodes so long as the voltage potential across them is less than a predetermined value, commonly referred to as the breakdown potential of the device. When the electrical potential across the gap exceeds the breakdown potential, the gas in the gap is ionized, producing a discharge current which flows across the gap between the electrodes thereby passing the induced electrical charge to ground and preventing the induced charge from reaching and damaging the equipment attached to the electrodes.
The electrode spacing and the structure of the confronting electrode surfaces are designed in conventional spark gap devices so that it does not interfere with the operation of the equipment to which it is attached. Thus, while conditions are normal, the conventional spark gap device is an insulator or at least is a very poor conductor, so that it prevents flow of system current through it. When a transient voltage appears that might exceed the strength of the insulation to be protected, the spark gap quickly becomes a good conductor and thus a good bypass for the surge current. The conventional, self-restoring spark gap device may be looked upon as a very fast switch or circuit breaker connected around the insulation to be protected. It is a breaker that is normally open but is able to close immediately when a transient voltage of a predetermined magnitude appears, and then is able to re-open speedily after a transient voltage has disappeared.
A problem associated with the use of a conventional self-restoring spark gap for protection of the field winding of a synchronous motor is that the gap can arc without operator knowledge. An arc will burn away the gap electrodes and increase the spacing which will change the calibration of the gaps. If the contacts burn enough to appreciably widen the gap, the field insulation of the machine's rotor may not be able to withstand the arc overvoltage required to breakdown the larger gap. Furthermore, such self-restoring spark gaps do not provide a non-reversible signal of malfunction in the field winding connections or in the control circuitry.